Hybrid powertrains typically include an engine, a transmission, an electric motor/generator and a high voltage battery. The transmission is coupled to the engine as is well known. The engine may produce an engine torque, which is supplied to the transmission. Alternatively, the electric motor/generator may convert an electric current from the high voltage battery into a torque, which is supplied to the transmission. The transmission includes a plurality of gears, operable to achieve a plurality of gear ratios as is well known. The transmission converts the torque from the engine and/or the electric motor/generator into an output torque, which is supplied to the wheels to power the vehicle. Excess torque from the engine, not needed to power the vehicle may be used by the electric motor/generator to generate electricity to charge the high voltage battery.
In operation, the high voltage battery is typically only charged up to a pre-defined upper limit. In other words, the high voltage battery should not be charged further once achieving the pre-defined upper limit. Continuous or further charging of the high voltage battery above and beyond the pre-defined upper limit may damage the high voltage battery.
When the high voltage battery is at or near the pre-defined upper limit, the hybrid powertrain may disconnect a fuel flow to the engine to disengage operation of the engine and cease engine torque production. Disengaging the operation of the engine prevents any additional charging of the high voltage battery. When the engine is operating in a condition in which the fuel flow to the engine may be disconnected, such as to prevent overcharging of the high voltage battery, then the engine is operating in a fuel-off enabled condition, i.e., the ability to prevent fuel flow to the engine is enabled. However, certain vehicle conditions exist in which the operation of the engine should not be disengaged because the high voltage battery may not have the ability to restart the engine when needed. For example, if the vehicle is traveling at or below a minimum speed, or if the temperature of the high voltage battery is below a minimum temperature, then the engine should not be disengaged. When the engine is operating in a condition in which the fuel flow to the engine should be maintained in order to satisfy an existing vehicle condition, such as when the electric motor may not be able to engage or otherwise re-start the engine, then the engine is operating in a fuel-off disabled condition, i.e., the ability to prevent fuel flow to the engine is disabled.
When the high voltage battery is at or above the pre-defined upper limit, and the hybrid powertrain is operating in the fuel-off disabled condition such that the fuel flow to the engine is maintained, then the excess torque produced by the engine is transferred to the transmission causing output torque spikes. The output torque spikes are undesirable, and may cause the vehicle to buck.